1. Field of the Invention
The invention relates in general to a voltage-clamping device, and more particularly to a voltage-clamping device capable of reducing the substrate current and negative effects generated in a transistor of an operational amplifier.
2. Description of the Related Art
Liquid crystal display comprises a source driver, a gate driver and a liquid crystal display panel, wherein the liquid crystal display panel has a pixel array. The source driver provides pixel data to columns of the pixel array and the gate driver turns on corresponding rows of the pixel array to display the desired image. Referring to FIG. 1, a curve of bias voltage vs. substrate current of a transistor of an operational amplifier is shown. According to most conventional technologies, the output buffer of the source driver is implemented by an operational amplifier, and a part of transistors will generate a big substrate current and result in hot electrons due to the high cross-voltage between the gate and the source and that between the drain and the source.
A part of hot electrons flow to the gate of the transistor and is trapped in the gate oxidation layer, causing the threshold voltage, saturation current and the characteristic of the transistor to change with the using time and end up with malfunction. As a result, the conventional operational amplifier and the source driver using the same have the disadvantage of shorter lifespan.
As higher substrate current would also increase the working current of the direct current of the conventional operational amplifier and the source driver using the same, the conventional operational amplifier and the conventional source driver using the same have the disadvantage of high power consumption. When the conventional operational amplifier and the conventional source driver using the same are used in portable products, the portable products will have poor battery life.
Higher substrate current would increase the substrate voltage so as to forwardly turn on the parasitic transistor thereof. Therefore latchup effect or snapback effect would be aroused. As a result, the substrate current increases drastically and more hot electrons are generated, and the above disadvantages are made even worse.